A. Field of the Invention
The present invention relates to the field of ophthalmic instruments, and more particularly to hand-held non-contact tonometers intended for xe2x80x9cat homexe2x80x9d self-testing by patients and office use by general medical practitioners.
B. Description of the Prior Art
Ophthalmologists and optometrists use non-contact tonometers to screen patients for elevated intraocular pressure associated with glaucoma. In the therapeutic care of glaucoma patients, ophthalmologists are confronted with the nearly impossible objective of assuring intraocular pressure control during the long time intervals between patient visits. Around-the-clock intraocular pressure monitoring studies cast serious doubt upon the relevance of sporadic observations in assessment of intraocular pressure control. Many medical doctors have expressed the need for simple and inexpensive non-contact tonometric instrumentation that could be used by patients at home, especially patients who have exhibited nerve tissue loss. Also, it is recognized that a simple and inexpensive non-contact tonometer designed for office use by general medical practitioners could improve chances for early diagnosis.
Hand-held tonometers of the xe2x80x9ccontactxe2x80x9d variety are well known, as exemplified by U.S. Pat. Nos. 4,192,317; 4,622,459; 4,747,296; and 5,174,292. For obvious reasons, these contact instruments are not suitable for self-measurement. Moreover, an operator""s skill in testing can have a significant impact upon measurement results, thus rendering these instruments poorly suited for use by general medical practitioners. Patient discomfort is also a drawback of direct contact tonometers.
U.S. Pat. No. 4,724,843 describes a portable non-contact tonometer that includes a carrying case 102 for housing a pump used to generate a fluid pulse, and a detachable hand-held unit 100 connected to the pump by a flexible connection line 104 enclosing a fluid conduit. The described non-contact tonometer precludes self-measurement because an operator other than the patient is required for alignment of the hand-held unit relative to the eye. In addition, the instrument itself is complex and expensive to manufacture.
It is therefore an object of the present invention to provide a non-contact tonometer that is suitable for home use by a patient. Portability, hand-held alignment, guidance in self-alignment, low cost, and readily understandable measurement output are desirable characteristics of a first embodiment of the present invention for meeting the stated object.
It is another object of the present invention to provide a non-contact tonometer that is suitable for office or clinic use by general medical practitioners. Portability, hand-held alignment, low cost, and readily understandable measurement output are desirable characteristics of a second embodiment of the present invention for meeting the stated object.
It is a further object of the present invention to introduce improved compactness into the design of non-contact tonometers by using a piston that moves directly along a measurement axis of the tonometer.
It is yet a further object of the present invention to provide an alignment guidance system for an ophthalmic instrument such as a tonometer that enables a patient to align the instrument in three-dimensions relative to an eye to be tested for self-testing purposes.
The hand-held non-contact tonometer according to the first embodiment of the present invention generally comprises a measurement axis along which a fluid discharge tube extends for discharging a fluid pulse toward the eye. The fluid discharge tube communicates with a fluid plenum that is subject to rapid volume decrease and pressure increase by a piston biased for movement along the measurement axis from a loaded position to an unloaded position. The piston itself includes a transparent plane parallel window normal to the measurement axis for transmitting collimated target and passive alignment light.
A patient is guided in self-alignment of the measurement axis and distancing the fluid discharge tube relative to a corneal pole by an alignment guidance system presenting a visible alignment image to a patient. The alignment guidance system includes a concave mirror facing the eye, and a configured target source originating at the measurement axis and residing in a plane normal to the measurement axis containing the center of curvature of the concave mirror. Divergent light from the target source is reflected by the concave mirror to form an inverted and reverted image of the target source about the measurement axis when alignment is achieved, thus appearing to the patient as a predetermined configuration centered on the measurement axis.
A passive alignment system is preferably incorporated into the tonometer to generate a signal indicating that three-dimensional alignment has been achieved for activating an electro-mechanical trigger mechanism to release the piston. An infra-red light source positioned at a focal point of the concave mirror is masked on one side to emit divergent rays toward the concave mirror, which reflects the incident light as a collimated beam along the measurement axis toward the eye. Light is focused at the front focal point of an objective lens. When the focal point coincides with the center of curvature of the cornea upon proper alignment, light is corneally retro-reflected through the system and diverted by a beam splitter through a lens focusing the light at a pinhole detector. A pair of masks having annular cut-outs prevent light from reaching the pinhole detector unless criteria for three-dimensional alignment are met.
Corneal applanation is detected according to known reflectance principles using obliquely incident light and a detector on opposite sides of the measurement axis to provide an applanation signal. A pressure sensor is arranged to monitor pressure within the fluid plenum and provide a corresponding pressure signal. The applanation signal and pressure signal are then evaluated to determine intraocular pressure, and one of three color-coded light-emitting diodes is illuminated based on the range of intracular pressuresxe2x80x94safe, borderline, or elevatedxe2x80x94into which the measured intraocular pressure falls.
The hand-held non-contact tonometer according to the second embodiment of the present invention is generally similar to that of the first embodiment, however the alignment guidance system, including the concave mirror, is removed and a target source is positioned remotely from the measurement axis to enable an operator to view through an eyepiece located along the measurement axis opposite from the objective lens. A visible spectral component of the retro-reflected target source light passes through a dichroic mirror on the measurement axis for focusing by the eyepiece, while an infra-red spectral component of the light is diverted by the dichroic mirror to a masked detector for passively confirming alignment. A digital readout is preferred to directly report the measured intraocular pressure to the operating medical practitioner.